EFFECT OF SINUS DENERVATION AND VAGOTOMY ON C-FOS EXPRESSION IN THE NUCLEUS-TRACTUS-SOLITARIUS AFTER EXPOSURE TO CO2

Exposure to hypercapnia and electrical stimulation of the carotid sinu s nerve (CSN) has been shown to induce c-fos expression in several bra in stem regions including the nucleus tractus solitarius (NTS). To tes t whether the labeled neurons were activated directly by hypercapnia o r secondarily via the carotid bodies (sinus nerve), adult rats were ex posed to either air or 14-16% CO2 for 1 h. Experiments were done on ei ght groups: (1) exposure to air, (2) exposure to CO?, (3) chronic CSN denervation/CO2, (4) chronic unilateral CSN denervation/CO2, (5) chron ic sham CSN denervation/CO2, (6) anesthetized/CO2, (7) anesthetized an d acute vagotomy/CO2, and (8) premedicated with morphine, 10 mg s.c., 20 min before exposure to CO2. After exposure to CO2 or air the rats w ere anesthetized, perfused with 4% paraformaldehyde and the brains pro cessed for immunohistochemical staining for c-fos protein using the PA P (i.e. peroxidase anti-peroxidase) technique. Labeled neurons in the area of the NTS in every second 50-mu m section were counted and their position plotted using a microscope and camera lucida attachment. Rat s exposed to CO2 had a significantly greater number of labeled neurons in the NTS than those exposed to air. Other interventions, such as CS N denervation, surgery anesthesia, vagotomy or injection of morphine d id not significantly affect the level of c-fos expression in rats expo sed to hypercapnia, indicative of central stimulation rather than seco ndary peripheral input. These responsive neurons may be part of a wide spread central chemoreceptive complex.